Mobile subscribers use wireless communication devices to communicate over a cellular wireless communication network by transmitting and receiving wireless signals between the wireless communication devices and one or more base stations, located at varying distances proximate to the various mobile subscribers.
Changing conditions, including movement of the mobile subscribers relative to the one or more base stations, will often necessitate that the wireless communication devices identify a different base station for facilitating further communication within the network. A decision to select a new serving base station, based upon the movement of the mobile subscriber relative to the one or more base stations, is often the result of a change in the received signal strength between the wireless communication devices and the previous serving base station. Either the received signal strength between the previous serving base station has degraded to an unacceptable level and/or a stronger signal is now available between the wireless communication device and a new alternative base station.
To insure that the wireless communication device is consistently communicating with the best available serving base station, a wireless communication device will periodically scan for neighboring cells and corresponding base stations, and attempt to maintain a synchronization with at least a couple of the more likely handover candidates, based upon the power detected in the associated frequencies, and correspondingly the base station's presence on a neighbor list. In this way, when it becomes desirable to affect a handover, the mobile station has a head start with respect to making a transition to at least some of the more likely handoff candidates.
However in some instances, the relatively high power measurement in a frequency associated with a base station included in a neighbor list can be the result of interference created by another cell from a different network. In many such instances, the interference will negatively impact the ability of the mobile phone to synchronize to the base station in the neighbor list, which is associated with the frequency having the high power measurement. Generally, a base station with which the mobile station cannot be synchronized is likely to eventually be removed from the list of likely candidates. However a base station in the neighbor list whose synchronization has been lost is not always immediately removed from the neighbor list. Sometimes, the failed synchronization needs to have been detected over a span of several synchronization attempts. In other instances, there may be a delay or latency associated with the mobile station's failure to synchronize with the base station over the requisite number of attempts, and the removal of the base station from the neighbor list. During this time, the network may send a handover command instructing the mobile station to attempt a handover to the base station for which synchronization has been recently lost.
In some instances, even though the synchronization has been lost, the mobile station will still attempt to execute a handover to the unsynchronized base station. This is sometimes referred to as a blind handoff, where in some instances, the mobile station may attempt to establish a handover to the unsynchronized base station for a predefined period of time, which in some instances can be on the order of up to 300 ms. In the event that the handover to the unsynchronized base station for which synchronization had been recently lost fails, in at least some of these instances the mobile station may attempt to reconnect to the base station through which the mobile station was previously connected. However, in many instances the mobile station may not be able to reconnect through the previous base station, in part due to the same reasons that the network made the decision to switch away from the previous base station. In such an instance, the mobile station may temporarily lose its connection to the network, and if the mobile station was in the middle of a call, the mobile station can sometimes drop the call.
Consequently, the present inventors have recognized, that more reliable handovers can occur if handover attempts under conditions likely to result in a failed handover attempt are avoided, which in turn can help to improve call performance.